Quantum mechanics dictates that electrons making transitions between allowed energy levels gain or lose a specific quanta of energy. In particular, the transition of an electron to an excited state is associated with the absorption of a quanta of energy and the transition of an electron to a state of lower energy produces a quanta of energy. In the latter case in which electrons make downward transitions to a lower energy level, energy is released in the form of both crystal lattice vibrations, i.e., photons, and emitted light photons. The lattice vibrations are manifested as heat, while the photons are observable as light energy, which, if in the visible spectrum and of sufficient intensity, can be detected by the human eye.
Light emitting diodes (LEDs) are well known semiconductor devices which convert electrical energy into emitted light. In a LED, the generation or injection of a current across the LED's diode junction followed by the recombination of the injected carriers encourages such electronic transitions and their resulting productions of vibrational energy or light, or both.
In these electronic transitions, it is well known that the wavelength of the light produced is controlled by the energy difference of the electronic transition. Accordingly, smaller electronic transitions within the visible spectrum, tend to emit longer wavelengths of light toward the red portion of the visible spectrum, and larger energy transitions tend to emit shorter wavelengths of light toward the violet portion of the visible spectrum. Such transitions are used to characterize the materials in which they occur, thereby enabling materials to be identified by the specific manner in which they behave under incident electromagnetic radiation, including visible, ultraviolet and infrared light. Hence, the peak wavelengths and associated colors of the luminescence generated by a given type of semiconductor material are restricted to a select few. For various physical and technological reasons, it has been more difficult to produce LEDs which emit shorter wavelengths of light toward the violet portion of the spectrum, including blue LEDs.
Since blue is one of the primary colors, the limited availability of inexpensive efficient blue LEDs creates limitations in related technological fields.
It is known that, theoretically, blue light can be produced by any semiconductor material having a band gap of at least .about.2.5 electron volts (eV). The band gap of a given semiconductor material is the energy difference between the conduction band and the valence band of the semiconductor material. Consequently, blue light emitting semiconductor diodes must be formed from a relatively large band gap semiconductor material such as gallium nitride (GaN), zinc selenide (ZnSe) or alpha silicon carbide (.alpha.-SiC).
The blue light produced by some of these semiconductor materials can be made more intense and more efficiently, by making the semiconductor material porous using well known anodizing and photoanodizing techniques. Some porous semiconductor materials exhibit unique optical properties which their bulk semiconductor counterparts can not equal. An example of this superiority is found in porous SiC which has exceptional optical and unique electronic properties due to its geometry. More specifically, porous SiC exhibits visible transparency and intense blue photoluminescence and electroluminescence. In terms of developing optoelectronic devices from porous semiconductor materials, some progress has been made in developing porous Si-based light emitting devices. Since the oxidation rates of bulk SiC and porous SiC are much lower than that of bulk Si and porous Si, and since SiC has been identified as a material for use at high temperatures, optoelectronic devices based on porous SiC will be much more stable over longer periods of time, and also at higher temperatures than those based on porous Si.
Another unique optical property of porous semiconductor materials, one which is employed in the current innovation, is that the porous semiconductor layer can be prepared so that its optical index of refraction varies spatially, in a periodic or continuously varying manner, or otherwise, due to a change in its porosity. With this, such a structure can function as an optical waveguide or optical interference filter.
Films of SiC are made porous by electrochemically fabricating a microcrystalline porous structure from bulk SiC with pore spacings of "quantum" dimensions (less than 10 nm). The resulting porous structure provides a large internal surface. The porous structure can be fabricated using either dark-current or light-assisted electrochemical means as disclosed in U.S. Pat. No. 5,376,241 entitled FABRICATING POROUS SILICON CARBIDE and U.S. Pat. No. 5,454,915 entitled METHOD OF FABRICATING POROUS SILICON CARBIDE (SIC), both of which were issued to Joseph S. Shor and Anthony D. Kurtz on Oct. 3, 1995 and assigned to Kulite Semiconductor Products, Inc., the assignee herein. Using these techniques to make films of SiC porous, it is possible to increase the quantum efficiency of SiC, resulting in UV or deep blue luminescence. Such porous SiC films exhibit a spectrally integrated photoluminescence intensity (and efficiency) which is approximately twenty (20) times higher than that which is observed from bulk Si, as discussed in Jonathan E. Spanier, et al., "Effects of Nanocrystalline Structure and Passivation on the Photoluminescent Properties of Porous Silicon Carbide", Mat. Res. Soc. Symp. Proc. Vol 452, pp 491-496, (1997). Accordingly, devices fabricated from porous SiC using existing processing techniques such as those described in U.S. Pat. Nos. 5,376,241 and 5,454,915, enable the development of semiconductor UV and blue light source and UV/blue optoelectronic devices.
The luminescence in the blue range of the spectrum can be further enhanced by passivating porous SiC with a passivation agent such as oxygen or hydrogen. Passivation enables the microcrystalline structures to satisfy the conditions for quantum confinement by preventing surface recombination at the dangling bond. Passivating agents that may be employed for this purpose include atomic hydrogen, deposited by a plasma or by an HF dip, oxygen, formed by thermal oxidation, anodically, or PECVD (plasma enhanced chemical vapor deposition) of oxygen, or any other passivating agent which will complete the dangling bond sites.
Initial demonstrations of room temperature luminescence from porous semiconductor materials such as porous SiC, created much speculation about the mechanisms which provide visible luminescence. However, it is now generally agreed, based on considerable theoretical and experimental evidence, that, in some cases, at least a portion of the enhancement of the luminescence is associated with the scale of the quantum structures in the porous semiconductor material. These quantum structures may allow a relaxation of the momentum selection rules by confining the charge carriers spatially, thus allowing direct band-gap transitions. Additionally, it has been demonstrated in porous silicon that the quantum confinement of charge carriers increases the effective band-gap, thereby pushing it into the visible region.
It is also generally agreed that the surface chemistry in porous semiconductor materials plays an important role in luminescence. This suggests that luminescence in passivated porous semiconductor materials may have similar mechanisms as in bulk semiconductor materials like Si, which exhibit band-gap widening into the visible region when hydride species are formed on the surface. A portion of the visible luminescence of porous Si, for example, may be associated with silicon hydride (SiH). It is not positively known whether the hydrogen termination serves only to passivate the surface or whether there is a contribution to the luminescence by amorphous SiH. Nevertheless, it is very clear that silicon microcrystals having dimensions of less than 5 nm, exhibit band-gap widening and the above-described band-gap luminescence.
Accordingly, there is a need for an improved LED structure and method for making which exhibits electroluminescent emission in the deep blue portion of the visible spectrum.